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ZIRCONIUM
Natural Abundance, Stable Isobars Zr90, 51.46%, Mo92 Zr91, 11.23%, Mo94 Zr92, 17.11%, Mo96 Zr94, 17.40%, Ru96 aZr96, 2.80% The only charge material which has been used in the separation of zirconium isotopes is anhydrous zirconium tetrachloride. The average charge consists of 350 g ZrCl4 in a style X-5 stainless steel charge bottle. Zirconium tetrachloride is prepared by passing a mixture of chlorine gas and carbon tetrachloride vapor over zirconium oxide which is heated to a temperature of 500-550ºC. Zirconium oxide in a Vycor boat is placed in a Pyrex tube 10 cm in diameter by 2 m in length. The inlet end of the tube is reduced to 5 cm and connected to a 500 ml flask which is connected to a cylinder of chlorine gas. The flask is heated with an electric heating mantle and fitted with a separatory funnel to provide a drop-by-drop feed of carbon tetrachloride. An electric tube furnace placed near the inlet end serves to heat approximately one-third of the Pyrex tube, and the whole apparatus is placed near a fume hood in such a manner that the exhaust end of the Pyrex tube projects well into the hood. This removes gaseous reaction products as well as unreacted chlorine and carbon tetrachloride. During operation the carbon tetrachloride is dropped slowly into the heated flask, where it vaporizes and, together with chlorine gas from the cylinder, is passed over the heated zirconium oxide. As soon as ZrCl4 is formed it sublimes from the hot end of the tube and collects in the cool zone as a light fluffy powder. Periodically the zirconium tetrachloride is removed from the tube and quickly transferred to sealed containers for storage. The efficiency of the reaction is improved by stirring the zirconium oxide every hour or so to expose a fresh surface. Loosely plugging the exhaust end of the reaction tube with glass wool serves to keep atmospheric moisture from entering the tube and hydrolyzing the zirconium tetrachloride. When the reaction is complete, nitrogen is used to sweep the tube free of any unreacted chlorine. Approximately 700 g of ZrCl4 per day can be prepared by this method. Due to the relative scarcity of high purity zirconium, the unresolved charge material is recycled and recovered. The recovery of zirconium consists of washing the calutron components in nitric acid, precipitating zirconium hydroxide with ammonia, removing copper from the nitric acid solution by electrolysis, reprecipitating with ammonia, precipitating impurities from hydrochloric acid solution with hydrogen sulfide, extracting iron with diethyl ether, and finally precipitating zirconium hydroxide with ammonium hydroxide. The purified zirconium hydroxide is converted to zirconium oxide by slowly heating to 800ºC. Elemental zirconium has a low order of toxicity; however, the finely divided metal forms an explosive mixture in air. Hydrolysis of zirconium tetrachloride to form hydrogen chloride and zirconyl chloride presents a toxicity hazard. An additional hazard is phosgene which is produced by the chlorination reaction. Safe handling of zirconium tetrachloride requires the intelligent use of rubber gloves, safety glasses, and a fume hood with good exhaust ventilation. aZr96 is radioactive with a half-life of >2 × 1014 years. |
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